1. Field of the Invention
The subject invention relates to a spark plug for a spark-ignited internal combustion engine, and more particularly toward a spark plug having a fired-in suppressor seal pack between an upper terminal stud and a lower center electrode.
2. Related Art
A spark plug is a device that extends into the combustion chamber of an internal combustion engine and produces a spark to ignite a mixture of air and fuel. In operation, charges of up to about 40,000 volts are applied through the spark plug center electrode, thereby causing a spark to jump the gap between the center electrode and an opposing ground electrode.
Electromagnetic interference (EMI), also known as radio frequency interference (RFI), is generated at the time of the electrical discharge across the spark gap. This is caused by the very short period of high frequency, high current oscillations at the initial breakdown of the gap and at points of re-firings. This EMI or RFI can interfere with entertainment radio, 2-way radio, television, digital data transmissions or any type of electronic communication. In a radio for example, the EMI or RFI is usually noticed as a “popping” noise in the audio that occurs each time a spark plug fires. Ignition EMI is always a nuisance and in extreme cases can produce performance and safety-related malfunctions.
Levels of EMI emitted by a spark ignited engine can be controlled or suppressed by various methods. Commonly, EMI suppression of the ignition system itself is accomplished by the use of resistive spark plugs, resistive ignition leads, and inductive components in the secondary high voltage ignition circuit. A common type of resistor/suppressor spark plug used for the suppression of EMI contains an internal resistor element placed within the ceramic insulator between the upper terminal stud and the lower center electrode. While internal resistor/suppressor spark plug designs are well-known, practical considerations have frustrated the ability to integrate a resistor in small-sized spark plugs, for example those used in small engines and the like. The current trend toward compact engines in automotive applications further compounds this issue by calling for ever-smaller spark plugs with ever-increasing performance characteristics. In particular, the fairly large cross-sectional area required for the resistor inside of the insulator weakens the structural integrity of the ceramic material by creating a thin wall section precisely in the region of an insulator which is often highly stressed during assembly and installation. This diminished structural integrity is also a consideration when a loose, granular resistor material is cold-pressed into the insulator, and later hot pressed to produce the so-called “fired-in suppressor seal” pack. I.e., the thin wall sections are prone to bursting, especially during the cold-pressing operation.
Yet another consideration when attempting to down-size this type spark plug arises from the diminished dielectric capacity of the insulator in thin sections. Specifically, the ceramic insulator material is a dielectric. Dielectric strength is generally defined as the maximum electric field which can be applied to the material without causing breakdown or electrical puncture thereof. Thin cross-sections of ceramic insulator can therefore result in dielectric puncture between the charged center electrode and the grounded shell.
The prior art has recognized this problem and proposed a solution as reflected in U.S. Pat. No. 6,380,664 to Pollner, issued Apr. 30, 2002. A representation of this prior art construction is depicted in FIG. 4 of the subject application. In particular, Pollner forms the resistor portion of its spark plug with a taper to reduce its cross-sectional area toward the center electrode. While such a construction has some merit, it remains limited in applicability. For example, Pollner requires a two-piece center electrode assembly, namely a pre-trimmed, lower portion made of noble metal held in end-to-end abutting contact with an upper contact pin. The end-to-end configuration is particularly sensitive to vibration disturbances at the point of abutting contact. Also, the fragile design of Pollner's contact pin is susceptible to warpage during a hot-pressing assembly operation. Furthermore, the distance at which the end of the center electrode projects from the core nose of the insulator is established by the seat position of the center electrode within the seal. In the example of Pollner, the projection distance is controlled by seating the lower center electrode upon a step in the core nose, but not within the seal portion of the resistor pack. This can, therefore, lead to integrity issues with the seal and the seating of the center electrode. It also increases the geometrical complexity of the central passage extending longitudinally through the ceramic insulator. Manufacturing complexity is also increased in this design. And still further, Pollner teaches the desirability of sealing along a portion of the length of the contact pin, between the noble metal center electrode in the nose portion of the insulator and the larger diameter head of the metallic contact pin. We learn from Pollner's citation of the progenitor prior art that this sealing along a portion of the length of the contact pin is accomplished by special coating with boronization, aluminization, nitration, or siliconization to achieve a gas-tight bond. A similar in situ sintering of the precious metal center electrode within the insulator is also contemplated. As will be readily appreciated, this special coating process applied to the contact pin and/or the center electrode is labor-intensive and cost-additive. It is required in Pollner to achieve adequate gas-pressure sealing due to the problematic architecture of its spark plug.
Accordingly, there is a need for an improved method of integrating a resistor and seal pack inside the insulator portion of a spark plug, i.e., between the upper terminal stud and the lower center electrode, in which the structural integrity and dielectric strength of the ceramic insulator can be maintained in all applications, and in particular in applications requiring miniaturization of a spark plug geometry for small engines and the like.